Homogeneity of solid propellant compositions is essential for reliability in mechanical, chemical, and stability properties of the cured propellant grains. Instability of propellant grains can relate to a combination of mechanical and chemical property deficiencies due to improper mixing.
Mixing of high solids loaded ammonium perchlorate composite propellants to achieve homogeneity can be a time consuming costly, and hazardous procedure. The main disadvantages encountered in the preparation and mixing of these propellants are that their total liquid contents are generally very low and complete wetting of the solid oxidizer surfaces is slow and difficult to achieve. It is not unknown for a high solids loaded propellant containing ultrafine particles to require as much as three days time to properly incorporate all the solids into the final mix. When particulate solid oxidizers and fuels are subjected to the friction of the mixing blades in a high viscosity formulation the chance of a violent reaction being initiated is increased.
Cramer, U.S. Pat. No. 3,022,149 assigned to North American Aviation, Inc., discloses a "quick mix technique" which suggests that various processing fluids, such as hexane or Freon--113, be used to lower the overall mix viscosity and to help in wetting the surfaces of the solid ingredients which in order to lower labor and processing time. Wetting generally requires the displacement of adsorbed air and moisture from the surface of the solid particulates.
Further use of processing fluids, namely organic liquids in combination with a surfactant for the dispersion of fine ammonium perchlorate is taught in U.S. Pat. No. 3,745,078, issued to Bernard J. Alley et al. on July 10, 1973 and assigned to the United States of America as represented by the Secretary of the Army. The ammonium perchlorate must be insoluble in the organic liquid and the surfactant must be soluble in the organic liquid to achieve proper wetting to achieve desired results. After the functional benefit is derived, the processing fluids have to be removed. Complete removal of processing fluids is a drawback. Other, more serious drawbacks of most of these processing fluids are their high flammability and toxic natures. In addition, wide variances in final propellant ballistic and mechanical properties are likely to be encountered when using these types of processing aids if the processing aids are not completely removed. Chemical analyses periodically performed towards the end of the mixing procedure are usually required to provide assurance of complete processing fluid removal. This coupled with the difficulty of completely removing liquid processing aids complicates the process and increases processing time and costs.
Significant improvements in the processing of high burn rate, high solids loaded ammonium perchlorate propellants have been attained through the incorporation of aziridine ingredients which are added during the initial grinding of ammonium perchlorate crystals. Aziridines were found to reduce the rate of crystal growth and agglomeration of fine (less than 1 micrometer) ammonium perchlorate particles. Aziridines also were found to improve bonding of the binder matrix to the particles. These improvements in propellant processing and mechanical properties have been attributed to the surfactant properties of these materials. However, even under the best of conditions, the mixing procedure is still lengthy, costly, and hazardous.
An improvement in the method of mixing high solids loaded propellants to decrease labor costs and mixing time is a very desirable feature; however, an improvement which achieves not only a decrease in labor costs and mixing times, but derives other benefits such as mixing of the propellant in an inert atmosphere at lower temperature and lower viscosity conditions, would be welcomed results derived from an innovation to the propellant processing and mixing art.
An object of this invention is to provide a method of enhancing the mixing and processing of high solids loaded ammonium perchlorate composite propellants in a completely inert and non-flammable solvent system.
A further object of this invention is to provide a method of mixing and processing high solids loaded ammonium perchlorate composite propellants at a lower temperature which can provide extended propellant pot life.
Still, another object of this invention is to provide a method of mixing and processing high solids loaded ammonium perchlorate composite propellants wherein the mixing and processing are achieved with a near critical liquid which is easily removed after it completes its function by undergoing a liquid to gas phase change.